1. Field of the Invention
The present invention relates to a filament for an X-ray tube, and more specifically to a coiled filament with an improvement in temperature distribution uniformity along the longitudinal direction of the filament. The present invention also relates an X-ray tube having such a filament. The present invention further also relates to an X-ray tube with an improvement for a longer lifetime of the filament.
2. Description of the Related Art
A coiled filament for an X-ray tube preferably gives itself a uniform temperature distribution as far as possible over the whole length of the filament. The ordinary coiled filament for an X-ray tube has a constant wire diameter and a constant coil pitch, and therefore its temperature becomes highest at the longitudinal center and drops in the vicinity of the both ends. If the temperature distribution of the filament is uniform, the intensity distribution of an electron beam emitted from the filament becomes uniform, so that the brightness distribution of an X-ray focus becomes uniform, the X-ray focus being made by the electron bombardment on the target (i.e., the anode) of an X-ray tube. In addition, if the temperature distribution of the filament is uniform, the amount of wire diameter wear of the coil becomes uniform as compared with a filament which is not uniform in temperature distribution, so that the lifetime is prolonged. Furthermore, if the temperature distribution of the coil is uniform, the maximum temperature of the filament can be lowered for obtaining the same X-ray tube current as compared with the filament which is not uniform in temperature distribution, so that the lifetime is prolonged as well.
While the present invention is concerned with a varied coil pitch of the filament for an X-ray tube, the prior art most relevant thereto is disclosed in Japanese Utility Model Publication No. 6-9047 U (1994), which will be referred to as the first publication.
The first publication discloses that a filament for an X-ray tube has a particular coil pitch which is dense in the vicinity of the center and sparse in the vicinity of the both ends, so that the temperature in the vicinity of the center of the filament rises to make the electron density distribution Gaussian. It is considered accordingly that the prior art filament does not make the temperature distribution uniform but rather makes the temperature in the vicinity of the center higher than the ordinary coil having a constant coil pitch. The coiled filament of the first publication is 80 turns per inch in coil pitch in the vicinity of the center and 50 turns per inch in the vicinity of the both ends for example.
On the other hand, in the technical field other than the X-ray tube, a coiled filament having a particular coil pitch which is sparse in the vicinity of the center and dense in the vicinity of the both ends so as to obtain a uniform longitudinal temperature distribution is known and disclosed in, for example, Japanese Patent Publication No. 63-232264 A (1988), which will be referred to as the second publication, and Japanese Utility Model Publication No. 1-161547 U (1989), which will be referred to as the third publication.
The second publication relates to a coiled filament of a halogen lamp for a copying machine and discloses a coiled filament having a particular coil pitch which is denser at the both ends than the central region so as to prevent temperature drop at the ends to make the luminance at the ends the same as the central region. For example, the coil pitch is 26.3 turns per centimeter at the central region and 33.8 turns per centimeter at the ends.
The third publication relates to a coiled filament for a lamp for use in such as a vehicle and discloses a coiled filament having a particular coil pitch which is sparser at the central region than the both ends so as to obtain a uniform longitudinal temperature distribution. The third publication also discloses that the coil pitch of the outermost turn is set to be densest and the coil pitch is expanded one by one from the outermost end toward the central region.
It would be understood from the second and third publications that if the coil pitch in the vicinity of the both ends of the coiled filament is set to be denser than the central region, the longitudinal temperature distribution of the filament becomes uniform. Then, on the basis of such an understanding, the inventors of the present invention have developed a coiled filament for an X-ray tube. It has been found, however, that only such an improvement is not sufficient for a good uniformity of the temperature distribution.
The temperature distribution of the X-ray tube filament affects the density distribution of the electron beam which is emitted from the filament, and the density distribution further affects the brightness distribution of the X-ray focus on the target. If it is desired only to prolong the lifetime of the filament, the use of the prior art disclosed in the second or third publication would be sufficient. But, taking account of the uniformity of the X-ray focus brightness too, a more precise uniformity of the temperature distribution is required.
Next, the lifetime of the filament will be discussed. A component which has the shortest lifetime in the X-ray tube is a filament. If the lifetime of the filament is prolonged, a maintenance cost and time for the X-ray tube can be greatly saved. The major factors affecting the lifetime of the filament are nonuniformity of the longitudinal temperature distribution of the filament and bombardment of ions coming from the target.
First, there will be explained the reduction of the lifetime caused by the nonuniformity of the longitudinal temperature distribution of the filament. Since the ordinary coiled filament for an X-ray tube has a constant wire diameter and a constant coil pitch, its temperature becomes highest at the longitudinal center and drops in the vicinity of the both ends. The filament is greatly wasted at the region which is higher in temperature, and thus the wire diameter is reduced at the higher-temperature region. When the wire diameter is reduced, the electric resistance is increased to raise the heating value at the region, resulting in a much higher temperature. Under such a vicious circle, the filament is finally broken at the higher-temperature region.
Next, there will be explained the reduction of the lifetime caused by the bombardment of ions coming from the target. The filament emits an electron beam which is narrowed by an electric field made by the Wehnelt electrode to make a specified electron-beam-irradiated region on a target, so that the irradiated region generates X-rays. The electron-beam-irradiation region emits not only X-rays but also metal atom ions, i.e., positive ions, the metal atom making up the target material. The ions may occasionally collide with the filament. When the filament experiences the ion bombardment, the filament is subject to erosion disadvantageously, resulting in the filament breaking at last.
The two problems regarding the lifetime reduction may be overcome separately with the suitable countermeasures which may be found out from the prior art.
First, in the field other than the X-ray tube, a coiled filament having a particular coil pitch which is sparse in the vicinity of the center and dense in the vicinity of the both ends so as to obtain a uniform longitudinal temperature distribution is known and disclosed in the second and third publications as mentioned above.
Next, in the field of the X-ray tube, the countermeasures in which the position of the filament is shifted from the position facing the electron-beam-irradiation region is known and disclosed in Japanese patent publication No. 5-242842 A (1993), which will be referred to as the fourth publication, and Japanese patent publication No. 2001-297725 A, which will be referred to as the fifth publication.
Each of the fourth and fifth publications discloses a combination of a couple of the eccentric filaments. The opening of the Wehnelt electrode is formed asymmetric about the filament so that the electron-beam-irradiation region on the target can be deviated from the filament center extension line. As a result, the filament becomes less subject to the ion bombardment.
The inventors of the present invention have been dedicated to make a study on elongation of the lifetime of the X-ray tube filament and finally found out that it is most effective for the long lifetime of the filament to attain at the same time the both of (1) dissolving the nonuniformity (especially a higher temperature at the longitudinal central region than other regions) of the temperature distribution of the filament and (2) reducing the ion bombardment on the filament.